1. Field of the Invention
This invention relates to methods of and arrangements for decoding and recoding composite digital television signals. The invention is particularly, but not exclusively, concerned with the decoding and recording of composite digital PAL or NTSC television signals.
2. Description of the Prior Art
When a composite digital PAL television signal has been recorded on a digital video tape recorder, it may be required to reproduce the recorded signal, decode it to obtain the original luminance component Y and the two chrominance components U and V, for example for the purpose of displaying a picture on a television monitor for editing purposes, and then to recode the components Y, U and V, and to re-record the signal so produced. Using known PAL decoders and coders, each time the signal is decoded and recoded there is some degradation of the resulting television picture. Such degradation is cumulative and so if there are many generations of decoding and recoding, as may well occur in a complex editing process, the final television picture is substantially degraded.
This problem arises not from any inherent characteristic of the PAL system, but from that in the composite digital PAL television signal the components Y, U and V share a common frequency band, If it were possible completely to separate the components Y, U and V into isolated spectra in a decoder, this would permit perfect decoding, and on recoding the original coded signal could be perfectly reformed. In this case, decoding and recoding could be repeated as often as required without consequent picture degradation.
In practice, however, complete separation of the components Y, U and V is not economically possible as it would involve very complex or so-called multi-dimensional filtering, which not only performs separation on the normal frequency basis, but also on the basis of frequency in the television picture height direction and on the basis of time.